The present invention relates to controlled-release and delayed delivery compositions.
For years manufacturers have sought ways to control the release of certain agents into various selected environments where they are to be introduced. These agents include, for example, medicines, pesticides, herbicides, cosmetics, laundry products, pigments, and many other materials. For these agents a delayed release and/or continuous release is in some cases desirable in order to maximize the agent's effectiveness, minimize or eliminate undesirable effects, or do both.
One of the most effective and currently wide-spread means of achieving the goal of controlled agent release is encapsulation. Encapsulation involves the formation of a protective wall of some type around a small particle, agglomeration of particles, or droplet of agent material. The wall is composed of a material suitable to achieve this goal, the material varying according to the degree of permeability needed, the type of undesirable reactions to be avoided, and any number of other requirements, each of which must be considered to ensure the best wall material choice. The agent and wall materials can together comprise solid and liquid mono-nuclear structures, emulsion and suspension compositions, and solid/liquid combinations in which a liquid is adsorbed into a porous matrix, as well as polynuclear structures involving solids, liquids, or both, and having more than one wall or agent layer.
To prepare capsules of these types a wide variety of methods and processes have been developed. One method involves the use of fluidized beds. In this process particles of agent material are sprayed with wall material while they are suspended in a gas stream, e.g., air or nitrogen. The wall material sprayed can be of a polymer solution, a molten wax, molten sulfur, an emulsion, a suspension such as a latex, or other material, and is continued until the desired wall thickness is obtained. A design modification called the Wurster column can be employed to reduce particle agglomeration. Both liquids and solids can be encapsulated using the fluidized bed method, with liquids generally requiring absorption into a porous matrix such as clay, freezing and/or drying, or the use of thickening additives either before or during the fluidization and coating steps. Regardless of the method or materials chosen, the goal is to produce a composition capable of controlled agent release, such as release at a predetermined time. This release can be immediate upon introduction into a selected environment, it can be delayed for a specific amount of time, or in the case of a number of agent particles or drops having differing wall thicknesses or compositions, it may be continuous over a period of varying length. The release is effected by varying mechanisms acting on the wall, such as by dissolution by the environment, reaction with the environment, or osmotic diffusion causing rupture of the capsule wall. U.S. Pat. No. 3,952,741, for example, illustrates a controlled delivery system based on osmotic bursting of a water-permeable wall.
These mechanisms are effective for a wide variety of uses. The osmotic rupture mechanism is particularly well-suited to applications where release of the agent is to be delayed for a predetermined amount of time and then effected fairly rapidly. One such application is, for example, in laundry products containing bleaches. Fabric damage can result when the bleach is allowed to dissolve prior to the completion of the washer's filling cycle and the start of the agitation cycle because of local high bleach concentrations. Therefore, in order to prevent the local high concentrations it is desirable to delay bleach release for a few minutes and then effect release rapidly. U.S. Pat. No. 3,992,317 describes an encapsulated composition, applied to peroxygen compounds, that accomplishes this delay. However, while release of a single particle of peroxygen compound may be effected rapidly, a graph of the release of a sampling of a number of particles prepared by the method of this patent shows a range of release times over a significant portion of the total time between introduction into the environment and complete release of all agent. These essentially sequential releases allow for the undesirable local high concentrations, although the concentrations will be somewhat more uniform than when no delay mechanism is used.
Similar problems are encountered when the type of encapsulation described in U.S. Pat. No. 3,992,317 is used for pesticides, herbicides, medicines, pigments, and so forth. Thus, what is needed is a composition and method for increasing the uniformity of release time for a given quantity of agent into a selected environment. The present invention is such a composition and method.